Agent86
Please drop your smart ass attitude, we don't want it here.
If you were more proactive and read quite recent forum posts you would find references to smartctl, which is part of sme server and can be used to test your drives in situ. So do a search on smartctl for the answers you are after.
As well you should obtain the drive manufacturers diagnostic software and run extensive tests on your suspected faulty drive, also in situ. You really should test both drives while you are at it. Only you know the brand of drives you use, so search Google.
You can download the Ultimate Boot CD (UBCD) which has lot's of test software on it. You would have also found this information pretty easily too with a Advanced forum search.
The drive is either faulty and that's why the array is broken, or there is some incompatibility with either the drive & RAID1, or with the motherboard and drive speeds. A while back there were issues with Seagate drives not staying in sync. All of this has been reported in the forums over the last couple of years, so as your knowledge is poor (as stated by you), then I suggest you set aside a few hours for some backreading of forum posts. You will learn a lot.
Also see
http://wiki.contribs.org/Raidand particularly here
http://wiki.contribs.org/Monitor_Disk_Healthboth of which you could have easily found without me having to tell, but you do seem a bit lazy.
Ever heard of the term "self help", I suggest you use it more often.
You seem to have some sort of arrogance attitudinal problem where you treat people here as hecklers. Grow up and behave like an adult, this is a skilled technical user forum, your poor responses and attitude have no place here, so please apologise and respond more politely in future.